Sean Travis was a former sonar operation who had served on a few attack subs before being dishonorably discharged.
Travis was an eco-terrorist and a member of Joshua Fox's anti sonar group which was dedicated to ensuring that the Navy stopped using sonar waves which were ultimately confusing the whales and eventually resulting in their deaths.
As such, Travis sought revenge against the United States Navy.
Driven by the need to complete his goal at any cost, Travis killed the real Petty Officer Drew by fracturing his skull with a hard object before dumping the body into a barrel of acid which left the remains so horribly disfigured that they couldn't even be identified at first.
The body was later discovered by two workmen, prompting NCIS Special Agent Timothy McGee who was stationed at the Norfolk Naval Base to call in the Major Case Response Team led by Leroy Jethro Gibbs from Washington D.C to help him investigate.
Having secured the ID he needed, Travis, now posing as Drew began operating as normal. In his possession, though, was a tiny canister designed to release sarin gas if it became cold enough to do.
In the meantime, Gibbs and Agent Caitlin Todd arrived on the Philadelphia to interview the five suspects, one of whom they suspected could have murdered the victim and stolen his identity.
Drew/Travis was among the suspects but kept his cool, playing his role to such a degree that it almost seemed to work.
After the submarine did an emergency blow, Drew/Travis, realizing that he was about to get arrested requested permission to make a head call which was given to him.
Fearing that he could get exposed and arrested, Drew/Travis then committed suicide by placing a plastic bag over his head which caused him to suffocate while also knowing that after his death, the crew of the Philadelphia would put his body in the freezer which was SOP on a sub which in turn would result in the sarin gas from the canister to be released, killing the entire crew including Gibbs and Kate.
Luckily, Gibbs caught onto his scheme and with the help of the rest of the crew, was able to eject Drew/Travis's body out of a torpedo tube, saving everyone from death while finally putting an end to Drew/Travis's plans for good.
Travis was an eco-terrorist and as such, was highly to avenging the whales so much so that he killed Petty Officer Drew (Sub Rosa) and took over the dead man's identity while keeping up the pretence that everything was alright.
Upon realizing that he would be arrested after being exposed, Travis committed suicide not because he had chosen to surrender but as a means of a back-up plan while also hoping that the cansister he had swallowed while contained sarin gas would eventually leak out and kill the crew of the USS Philadelphia but thanks to Gibbs's efforts, the plan later failed altogether.
- Drew (Sub Rosa)- (killed him by fracturing his skull with a metal object before dumping the body in a barrel of acid, impersonated).
- The crew of the USS Philadelphia- (attempted).
- Leroy Jethro Gibbs- (attempted).
- Caitlin Todd- (attempted).